The design of hypersonic vehicles is signiﬁcantly aﬀected by the state of boundary layer. Hypersonic boundary layers can be laminar or turbulent, and in chemical and vibrational nonequilibrium, each with diﬀerent length and time scales. In turbulent boundary layers, heating augmentation can be an order of magnitude or higher above laminar heating rates. The scales of the internal energy relaxation processes can be of the same order or greater than the turbulent ﬂow scales, and can interact with turbulent motion. Understanding how turbulent motion and internal energy relaxation interact is relevant to ﬂow control. Fundamental ﬂows are studied to understand how energy is exchanged between turbulent motion and internal energy relaxation. Speciﬁcally, high-ﬁdelity DNS of vibrational energy relaxation eﬀects in compressible isotropic and temporally evolving shear layers are presented. The energy exchange mechanisms are analyzed by decomposing the ﬂow into incompressible and compressible energy modes. By varying the vibrational relaxation rate, the tuning of the relaxation rate to the turbulent ﬂow is studied. Vibrational energy relaxation is demonstrated to be coupled to the turbulent ﬂow through the compressible modes of the gas. Compressions and expansions generate ﬂuctuations in the thermal state, and the vibrational energy lags behind these ﬂuctuations. Energy is then transferred to or from the vibrational energy mode at a rate proportional to the relaxation time, and the ﬂuctuations are damped. Damping of turbulent quantities are strongest when the vibrational relaxation rate is on the order of the turbulent large structure acoustic rate. Wavenumber speciﬁc damping is also observed in isotropic ﬂows when the relaxation time is on the order of the acoustic frequency of the wave. The eﬀects of vibrational relaxation are shown to increase with compressibility. However, the overall eﬀect on turbulent kinetic energy is weak due to the incompressible mode containing signiﬁcantly more energy than the compressible modes.
University of Minnesota Ph.D. dissertation. 2018. Major: Aerospace Engineering and Mechanics. Advisor: Graham Candler. 1 computer file (PDF); 125 pages.
Computational Analysis of Energy Exchange Mechanisms in Turbulent Flows with Thermal Nonequilibrium.
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